In general, this invention relates to radiant furnaces of the type used for firing ceramic material such as artificial teeth and the like. More particularly, it relates to a radiant furnace having an improved heating control system.
Electric furnaces have long been used in the dental industry in connection with the manufacture of artificial teeth. The manufacturing process typically includes an initial step of making an impression of a patient's mouth. Then a plaster model is made and tested for proper fit and other purposes. A selection is then made of the most appropriate ceramic material (or material) from which the artificial tooth will be made so that it will closely resemble the patient's natural teeth. The selected ceramic material is then crafted to conform to the required size and shape. Finally, the crafted ceramic material is dried, fired, and glazed.
A radiant furnace is used for firing and glazing the crafted ceramic material, and can be used for the preliminary drying as well. To this end, the interior of the radiant furnace has one region that is not directly exposed to the radiant heat and that is occupied by the crafted ceramic material during the preliminary drying; and another region, directly exposed to the radiant heat, where the ceramic material is fired and glazed. Preferably, an automatic carriage mechanism is provided for transporting the crafted ceramic material from the drying region to the firing region.
The firing step of the process occurs while the ceramic material is in the firing region. Typically, the ceramic material is heated until its temperature is brought up to or about a target temperature, and then the ceramic material is subjected to an air quench to cool it. Sometimes, an additional stage of operation is involved whereby the ceramic material is maintained at or about the target temperature for a preselected interval of time before it is cooled. In any case, it is desirable to provide a static type of temperature control to cause heating to stop when the ceramic material temperature is brought up to a temperature falling within a target temperature range. Prior art radiant furnaces have proven suitable with respect to providing such static temperature control. Experience has shown that consistent results require a dynamic type of temperature control. That is, it is important to provide accurate control over the rate of increase of the ceramic material temperature. Heretofore, heating systems in radiant furnaces have proven unsatisfactory with respect to ensuring that the rate of increase of temperature falls within acceptable limits. Undesirable variations in this rate of increase usually result from variations in the electric power supplied to the furnace. It is common for the line voltage in a given location to vary from as low as 100V rms to as high as 120V rms during a single day. Moreover, where the voltage applied to an electric heating element varies, the heating energy produced varies as a function of the square of the variations in applied voltage.